A family of highly conserved oxygen, iron, and 2-oxoglutarate-dependent prolyl hydroxylase (PHD) enzymes mediate the cells response to hypoxia via post-translational modification of hypoxia-inducible factors (HIF) (Ivan et al., 2001, Science, 292:464-68; Jaakkola et al., 2001, Science, 292:468-72). Under normoxic conditions, PHD catalyzes the hydroxylation of two conserved proline residues within HIF. As the affinity of PHD for oxygen is within the physiological range of oxygen and oxygen is a necessary co-factor for modifying hydroxylated HIF, PHD is inactivated when oxygen tension is reduced. In this way, HIF is rapidly degraded under normoxic conditions but accumulates in cells under hypoxic conditions or when PHD is inhibited.
Four isotypes of PHD have been described: PHD1, PHD2, PHD3, and PHD4 (Epstein et al., 2001, Cell, 107:43-54; Kaelin, 2005, Annu Rev Biochem., 74:115-28; Schmid et al., 2004, J Cell Mol Med., 8:423-31). The different isotypes are ubiquitously expressed but are differentially regulated and have distinct physiological roles in the cellular response to hypoxia. There is evidence that the various isotypes have different selectivity for the three different HIF-α sub-types (Epstein et al., supra). In terms of cellular localization, PHD1 is primarily nuclear, PHD2 is primarily cytoplasmic, and PHD3 appears to be both cytoplasmic and nuclear (Metzen E, et al. 2003, J Cell Sci., 116(7):1319-26). PHD2 appears to be the predominant HIF-α prolyl hydroxylase under normoxic conditions (Ivan et al., 2002. Proc Natl Acad USA, 99(21):13459-64; Berra et al., 2003, EMBO J., 22:4082-90). The three isotypes have a high degree of amino-acid homology and the active site of the enzyme is highly conserved.
Targeted disruption of the PHD enzyme activity by small molecules has potential utility in the treatment of disorders of oxygen sensing and distribution. Examples include but are not limited to: anemia; sickle cell anemia; peripheral vascular disease; coronary artery disease; heart failure; protection of tissue from ischemia in conditions such as myocardial ischemia, myocardial infarction and stroke; preservation of organs for transplant; treatment of tissue ischemia by regulating and/or restoring blood flow, oxygen delivery and/or energy utilization; acceleration of wound healing particularly in diabetic and aged patients; treatment of burns; treatment of infection; bone healing, and bone growth. In addition, targeted disruption of PHD is expected to have utility in treating metabolic disorders such as diabetes, obesity, ulcerative colitis, inflammatory bowel disease and related disorders such as Crohn's disease. (Recent Patents on Inflammation & Allergy Drug Discovery, 2009, 3:1-16).
HIF has been shown to be the primary transcriptional factor that leads to increased erythropoietin production under conditions of hypoxia (Wang et al., 1993, supra). While treatment with recombinant human erythropoietin has been demonstrated to be an effective method of treating anemia, small molecule mediated PHD inhibition can be expected to offer advantages over treatment with erythropoietin. Specifically, the function of other HIF gene products is necessary for hematopoesis and regulation of these factors increases the efficiency of hematopoesis. Examples of HIF target gene products that are critical for hematopoesis include: transferrin (Rolfs et al., 1997, J Biol Chem., 272(32):20055-62), transferrin receptor (Lok et al., 1999, J Biol Chem., 274(34):24147-52; Tacchini et al., 1999, J Biol Chem., 274(34):24142-46) and ceruloplasmin (Mukhopadhyay et al., 2000, J Biol Chem., 275(28):21048-54). Hepcidin expression is also suppressed by HIF (Peyssonnaux et al., 2007, J Clin Invest., 117(7):1926-32) and small molecule inhibitors of PHD have been shown to reduce hepcidin production (Braliou et al., 2008, J Hepatol., 48:801-10). Hepcidin is a negative regulator of the availability of the iron that is necessary for hematopoesis, so a reduction in hepcidin production is expected to be beneficial to the treatment of anemia. PHD inhibition may also be useful when used in conjunction with other treatments for anemia including iron supplementation and/or exogenous erythropoietin. Studies of mutations in the PHD2 gene occurring naturally in the human population provide further evidence for the use of PHD inhibitors to treat anemia. Two recent reports have shown that patients with dysfunctional mutations in the PHD2 gene display increased erythrocytosis and elevated blood hemoglobin (Percy et al., 2007, PNAS, 103(3):654-59; Al-Sheikh et al., 2008, Blood Cells Mol Dis., 40:160-65). In addition, a small molecule PHD inhibitor has been evaluated in healthy volunteers and patients with chronic kidney disease (U.S. Pat. App. No. US2006/0276477, Dec. 7, 2006). Plasma erythropoietin was increased in a dose-dependent fashion and blood hemoglobin concentrations were increased in the chronic kidney disease patients.
Overall accumulation of HIF under hypoxic conditions governs an adaptive up-regulation of glycolysis, a reduction in oxidative phosphorylation resulting in a reduction in the production of hydrogen peroxide and superoxide, optimization of oxidative phosphorylation protecting cells against ischemic damage. Thus, PHD inhibitors are expected to be useful in organ and tissue transplant preservation (Bernhardt et al., 2007, Methods Enzymol., 435:221-45). While benefit may be achieved by administering PHD inhibitors before harvesting organs for transplant, administration of an inhibitor to the organ/tissue after harvest, either in storage (e.g., cardioplegia solution) or post-transplant, may also be of therapeutic benefit.
PHD inhibitors are expected to be effective in preserving tissue from regional ischemia and/or hypoxia. This includes ischemia/hypoxia associated with inter alia: angina, myocardial ischemia, stroke, ischemia of skeletal muscle. Recently, ischemic pre-conditioning has been demonstrated to be a HIF-dependent phenomenon (Cai et al., 2008, Cardiovasc Res., 77(3):463-70). While the concept of pre-conditioning is best known for its protective effects in the heart, it also applies to other tissues including but not limited to: liver, skeletal muscle, liver, lung, kidney, intestine and brain (Pasupathy et al., 2005, Eur J Vasc Endovasc Surg., 29:106-15; Mallick et al., 2004, Dig Dis Sci., 49(9):1359-77). Experimental evidence for the tissue protective effects of PHD inhibition and elevation of HIF have been obtained in a number of animal models including: germ-line knock out of PHD1 which conferred protection of the skeletal muscle from ischemic insult (Aragonés et al., 2008, Nat Genet., 40(2):170-80), silencing of PHD2 through the use of siRNA which protected the heart from ischemic insult (Natarajan et al., 2006, Circ Res., 98(1):133-40), inhibition of PHD by administering carbon monoxide which protected the myocardium from ischemic injury (Chin et al., 2007, Proc Natl Acad Sci. U.S.A., 104(12):5109-14), hypoxia in the brain which increased the tolerance to ischemia (Bernaudin et al., 2002, J Cereb Blood Flow Metab., 22(4):393-403). In addition, small molecule inhibitors of PHD protect the brain in experimental stroke models (Siddiq et al., 2005, J Biol Chem., 280(50):41732-43). Moreover, HIF up-regulation has also been shown to protect the heart of diabetic mice, where outcomes are generally worse (Natarajan et al., 2008, J Cardiovasc Pharmacol., 51(2):178-187). The tissue protective effects may also be observed in Buerger's disease, Raynaud's disease, and acrocyanosis.
The reduced reliance on aerobic metabolism via the Kreb's cycle in the mitochondria and an increased reliance on anaerobic glycolysis produced by PHD inhibition may have beneficial effects in normoxic tissues. It is important to note that PHD inhibition has also been shown to elevate HIF under normoxic conditions. Thus, PHD inhibition produces a pseudohypoxia associated with the hypoxic response being initiated through HIF but with tissue oxygenation remaining normal. The alteration of metabolism produced by PHD inhibition can also be expected to provide a treatment paradigm for diabetes, obesity and related disorders, including co-morbidities.
Globally, the collection of gene expression changes produced by PHD inhibition reduce the amount of energy generated per unit of glucose and will stimulate the body to burn more fat to maintain energy balance. The mechanisms for the increase in glycolysis are discussed above. Other observations link the hypoxic response to effects that are expected to be beneficial for the treatment of diabetes and obesity. Hypoxia and hypoxia mimetics such as desferrioxamine have been shown to prevent adipocyte differentiation (Lin et al., 2006, J Biol Chem., 281(41):30678-83; Carrière et al., 2004, J Biol Chem., 279(39):40462-69). Inhibition of PHD activity during the initial stages of adipogenesis inhibits the formation of new adipocytes (Floyd et al., 2007, J Cell Biochem., 101:1545-57). Hypoxia, cobalt chloride and desferrioxamine elevated HIF and inhibited PPAR gamma 2 nuclear hormone receptor transcription (Yun et al., 2002, Dev Cell., 2:331-41). As PPAR gamma 2 is an important signal for adipocyte differentiation, PHD inhibition can be expected to inhibit adipocyte differentiation. These effects were shown to be mediated by the HIF-regulated gene DEC1/Stra13 (Yun et al., supra).
Small molecular inhibitors of PHD have been demonstrated to have beneficial effects in animal models of diabetes and obesity (Intl. Pat. App. Pub. No. WO2004/052284, Jun. 24, 2004; WO2004/052285, Jun. 24, 2004). Among the effects demonstrated for PHD inhibitors in mouse diet-induced obesity, db/db mouse and Zucker fa/fa rat models were lowering of: blood glucose concentration, fat mass in both abdominal and visceral fat pads, hemoglobin A1c, plasma triglycerides, body weight as well as changes in established disease bio-markers such as increases in the levels of adrenomedullin and leptin. Leptin is a known HIF target gene product (Grosfeld et al., 2002, J Biol Chem., 277(45):42953-57). Gene products involved in the metabolism in fat cells were demonstrated to be regulated by PHD inhibition in a HIF-dependent fashion (Intl. Pat. App. Pub. No. WO2004/052285, supra). These include apolipoprotein A-IV, acyl CoA thioesterase, carnitine acetyl transferase, and insulin-like growth factor binding protein (IGFBP)-1.
PHD inhibitors are expected to be therapeutically useful as stimulants of vasculogenesis, angiogenesis, and arteriogenesis. These processes establish or restore blood flow and oxygenation to the tissues under ischemia and/or hypoxia conditions (Semenza et al., 2007, J Cell Biochem., 102:840-47; Semenza, 2007, Exp Physiol., 92(6):988-91). It has been shown that physical exercise increases HIF-1 and vascular endothelial growth factor in experimental animal models and in humans (Gustafsson et al. 2001, Front Biosci., 6:D75-89) and consequently the number of blood vessels in skeletal muscle. VEGF is a well-known HIF target gene product that is a key driver of angiogenesis (Liu et al., supra). PHD inhibition offers a potential advantage over other angiogenic therapies in that it stimulates a controlled expression of multiple angiogenic growth factors in a HIF-dependent fashion including but not limited to: placental growth factor (PLGF), angiopoietin-1 (ANGPT1), angiopoietin-2 (ANGPT2), platelet-derived growth factor beta (PDGFB) (Carmeliet, 2004, J Intern Med., 255:538-61; Kelly et al., 2003, Circ Res., 93:1074-81) and stromal cell derived factor 1 (SDF-1) (Ceradini et al., 2004, Nat Med., 10(8):858-64). Expression of angiopoietin-1 during angiogenesis produces leakage-resistant blood vessels, in contrast to the vessels produced by administration of VEGF alone (Thurston et al., 1999, Science, 286:2511-14; Thurston et al., 2000, Nat Med., 6(4):460-3; Elson et al., 2001, Genes Dev., 15(19):2520-32). Stromal cell derived factor 1 (SDF-1) has been shown to be critical to the process of recruiting endothelial progenitor cells to the sites of tissue injury. SDF-1 expression increased the adhesion, migration and homing of circulating CXCR4-positive progenitor cells to ischemic tissue. Furthermore inhibition of SDF-1 in ischemic tissue or blockade of CXCR4 on circulating cells prevents progenitor cell recruitment to sites of injury (Ceradini et al., 2004, supra; Ceradini et al., 2005, Trends Cardiovasc Med., 15(2):57-63). Importantly, the recruitment of endothelial progenitor cells to sites of injury is reduced in aged mice and this is corrected by interventions that increase HIF at the wound site (Chang et al., 2007, Circulation, 116(24):2818-29). PHD inhibition offers the advantage not only of increasing the expression of a number of angiogenic factions but also a co-ordination in their expression throughout the angiogenesis process and recruitment of endothelial progenitor cells to ischemic tissue.
PHD inhibitors are useful in pro-angiogenic therapies, too. Adenovirus-mediated over-expression of HIF has been demonstrated to induce angiogenesis in non-ischemic tissue of an adult animal (Kelly et al., 2003, Circ Res., 93(11):1074-81) providing evidence that therapies that elevate HIF, such as PHD inhibition, will induce angiogenesis. Placental growth factor (PLGF), also a HIF target gene, has been show to play a critical role in angiogenesis in ischemic tissue (Carmeliet, 2004, J Intern Med., 255(5):538-61; Luttun et al., 2002, Ann N Y Acad Sci., 979:80-93). The potent pro-angiogenic effects of therapies that elevate HIF have been demonstrated, via HIF over-expression, in skeletal muscle (Pajusola et al., 2005, FASEB J., 19(10):1365-7; Vincent et al., 2000, Circulation, 102:2255-61) and in the myocardium (Shyu et al., 2002, Cardiovasc Res., 54:576-83). The recruitment of endothelial progenitor cells to the ischemic myocardium by the HIF target gene SDF-1 has also been demonstrated (Abbott et al., 2004, Circulation, 110(21):3300-05). Thus, PHD inhibitors will likely be effective in stimulating angiogenesis in the setting of tissue ischemia, particularly muscle ischemia. Therapeutic angiogenesis produced by PHD inhibitors will likely lead to restoring blood flow to tissues and therefore meliorate such diseases as but not limited to angina pectoris, myocardial ischemia and infarction, peripheral ischemic disease, claudication, gastric and duodenal ulcers, ulcerative colitis, and inflammatory bowel disease.
PHD and HIF play a central role in tissue repair and regeneration including healing of wounds and ulcers. Recent studies have demonstrated that an increased expression of all three PHDs at wound sites in aged mice with a resulting reduction in HIF accumulation (Chang et al., supra). Thus, elevation of HIF in aged mice by administering desferrioxamine increased the degree of wound healing back to levels observed in young mice. Similarly, in a diabetic mouse model, HIF elevation was suppressed compared to non-diabetic litter mates (Mace et al., 2007, Wound Repair Regen., 15(5):636-45). Topical administration of cobalt chloride, a hypoxia mimetic, or over-expression of a murine HIF that lacks the oxygen-dependent degradation domain and thus provides for a constitutively active form of HIF, resulted in increased HIF at the wound site, increased expression of HIF target genes such as VEGF, Nos2, and Hmox1 and accelerated wound healing. The beneficial effect of PHD inhibition is not restricted to the skin and small molecule inhibitors of PHD have recently been demonstrated to provide benefit in a mouse model of colitis (Robinson et al., 2008, Gastroenterology, 134(1):145-55).
In summary, PHD inhibition resulting in accumulation of HIF likely acts by at least four mechanisms to contribute to accelerated and more complete healing of wounds: 1) protection of tissue jeopardized by hypoxia and/or ischemia, 2) stimulation of angiogenesis to establish or restore appropriate blood flow to the site, 3) recruitment of endothelial progenitor cells to wound sites, 4) stimulation of the release of growth factors that specifically stimulate healing and regeneration.
As PDGF is a HIF gene target (Schultz et al., 2006, Am J Physiol Heart Circ Physiol., 290(6):H2528-34; Yoshida et al., 2006, J Neurooncol., 76(1):13-21), PHD inhibition likely increases the expression of endogenous PDGF and produces a similar or more beneficial effect to those produced with PDGF alone. Studies in animals have shown that topical application of PDGF results in increased wound DNA, protein, and hydroxyproline amounts; formation of thicker granulation and epidermal tissue; and increased cellular repopulation of wound sites. PDGF exerts a local effect on enhancing the formation of new connective tissue. The effectiveness of PHD inhibition is likely greater than that produced by PDGF due to the additional tissue protective and pro-angiogenic effects mediated by HIF.
The beneficial effects of inhibition of PHD extends not only to accelerated wound healing in the skin and colon but also to the healing of other tissue damage including but not limited to gastrointestinal ulcers, skin graft replacements, burns, chronic wounds and frost bite.
Stem cells and progenitor cells are found in hypoxic niches within the body and hypoxia regulates their differentiation and cell fate (Simon et al., 2008, Nat Rev Mol Cell Biol., 9:285-96). Thus, PHD inhibitors may be useful to maintain stem cells and progenitor cells in a pluripotent state and to drive differentiation to desired cell types. Stem cells may be useful in culturing and expanding stem cell populations and may hold cells in a pluripotent state while hormones and other factors are administered to the cells to influence the differentiation and cell fate.
A further use of PHD inhibitors in the area of stem cell and progenitor cell therapeutics relates to the use of PHD inhibitors to condition these cells to withstand the process of implantation into the body and to generate an appropriate response to the body to make the stem cell and progenitor cell implantation viable (Hu et al., 2008, J Thorac Cardiovasc Surg., 135(4):799-808). More specifically PHD inhibitors may facilitate the integration of stem cells and draw in an appropriate blood supply to sustain the stem cells once they are integrated. This blood vessel formation will also function to carry hormones and other factors released from these cells to the rest of the body.
PHD inhibitors may also be useful in the treatment of infection (Peyssonnaux et al., 2005, J Invest Dermatol., 115(7):1806-15; Peyssonnaux et al., 2008 J Invest Dermatol., 2008 August; 128(8):1964-8). HIF elevation has been demonstrated to increase the innate immune response to infection in phagocytes and in keratinocytes. Phagocytes in which HIF is elevated show increased bacteriacidal activity, increased nitric oxide production and increased expressed of the anti-bacterial peptide cathelicidin. These effects may also be useful in treating infection from burns.
HIF has also been shown to be involved in bone growth and healing (Pfander D et al., 2003 J Cell Sci., 116(Pt 9):1819-26., Wang et al., 2007 J Clin Invest., 17(6):1616-26.) and may therefore be used to heal or prevent fractures. HIF stimulates of glycolysis to provide energy to allow the synthesis of extracellular matrix of the epiphyseal chondrocytes under a hypoxic environment. HIF also plays a role in driving the release of VEGF and angiogenesis in bone healing process. The growth of blood vessels into growing or healing bone can be the rate limiting step in the process.
Small molecules inhibitors of PHD have been described in the literature, which include, but are not limited to, imidazo[1,2-a]pyridine derivatives (Warshakoon et al., 2006, Bioorg Med Chem Lett., 16(21):5598-601), substituted pyridine derivatives (Warshakoon et al., 2006, Bioorg Med Chem Lett., 16(21):5616-20), pyrazolopyridines (Warshakoon et al., 2006, Bioorg Med Chem Lett., 16(21):5687-90), bicyclic heteroaromatic N-substituted glycine derivatives (Intl. Pat. App. Pub. No. WO2007/103905, Sep. 13, 2007), quinoline based compounds (Intl. Pat. App. Pub. No. WO2007/070359, Jun. 21, 2007), pyrimidinetrione N-substituted glycine derivatives (Intl. Pat. App. Pub. No. WO2007/150011, Dec. 27, 2007), substituted aryl or heteroaryl amide compounds (U.S. Pat. App. Pub. No. US 2007/0299086, Dec. 27, 2007) and substituted 4-hydroxypyrimidine-5-carboxamides (Intl. Pat. App. Pub. No. WO2009/117269, Sep. 24, 2009).